In order to compete with conventional high-end film-based projection systems and provide what has been termed electronic or digital cinema, digital projectors must be capable of achieving brightness levels on the order of 10,000 lumens, projected onto screen sizes on the order of 40 feet in diagonal. The range of screens requires anywhere from 5,000 lumens to upwards of 40,000 lumens. In addition to this demanding brightness requirement, these projectors must also deliver high resolution (at least 2048×1080 pixels) and provide around a 2000:1 contrast ratio and a wide color gamut.
Some commercialized digital cinema projector designs have proved to be capable of this level of performance. However, there are still significant hurdles in usability and cost that must be overcome for widespread adoption of digital cinema systems. Digital projection apparatuses that meet the necessary performance requirements typically cost in excess of $50,000 each and utilize high wattage Xenon arc lamps that need replacement at intervals between 500-2000 hours, with typical lamp replacement cost often exceeding $1000.
One type of spatial light modulator (SLM) that is being widely utilized in projection solutions for multicolor digital cinema projection is the Digital Light Processor (DLP), a digital micromirror device (DMD), developed by Texas Instruments, Inc., Dallas, Tex. DLP-based projectors have demonstrated the capability to provide the necessary light throughput, contrast ratio, and color gamut for most projection applications from desktop to large cinema venues. DLP devices are described in a number of patents, for example U.S. Pat. No. 4,441,791; U.S. Pat. No. 5,535,047 and U.S. Pat. No. 5,600,383, all to Hornbeck. Significantly, DLP devices can be used with solid-state light sources, such as lasers. Lasers are known to be advantaged over other types of light sources with regard to relative spectral purity and potentially high brightness levels. Solid-state lasers promise improvements in etendue, longevity, and overall spectral and brightness stability. The use of lasers can significantly reduce the cost of the illumination system and, at the same time, offer increased color gamut and sufficient brightness levels compatible with digital cinema applications. Laser arrays of particular interest for projection applications include various types of VCSEL arrays, including VECSEL (Vertical Extended Cavity Surface-Emitting Laser) and NECSEL (Novalux Extended Cavity Surface-Emitting Laser) devices from Arasor, Sunnyvale, Calif.
Among problems that must be addressed when using laser illumination with DLP and other micromechanical devices are the generation of unused off-state or “dump” light and the corresponding heat generation that results from attempts to absorb the unused dump light. Off-state or dump light is an inevitable by-product of the DLP modulation scheme. In order to understand how off-state light is generated, it is first useful to trace the path of modulated light. Referring to the schematic diagram of FIG. 1, a projector apparatus 10 has Red, Green, and Blue light modulation assemblies 40r, 40g, 40b, each with a corresponding light source 12r, 12g and 12b, respectively. Light source 12r has a red laser or other low angle (f/6 or higher) solid-state red light source; light source 12g has a green laser or other low angle solid-state green light source; light source 12b has a blue laser or other low angle solid-state blue light source. The light path within each light modulation assembly 40r, 40g and 40b follows the same basic pattern. An illumination beam from light source 12r, 12g or 12b is directed through a lens 50, an integrator 51, and other conditioning optics onto a thin-film dichroic surface 68. Dichroic surface 68 is treated to reflect or transmit incident light according to its incident angle. Incident illumination is reflected from the dichroic surface 68 onto a micro-mirror spatial light modulator 60, such as a DLP device, that reflects modulated light back through dichroic surface 68 and to a dichroic combiner 82. The dichroic combiner 82 has an arrangement of dichroic surfaces 84 that selectively reflect or transmit light according to wavelength, combining the modulated light from each light modulation assembly 40r, 40g and 40b onto a single optical path through projection optics 70, which project images of the spatial light modulators 60 onto a projection surface (not shown).
For a projector arrangement such as projector apparatus 10 in FIG. 1, off-state or dump light is the unused light that is deflected such that it is not reflected along the path of modulated light for projection onto the projection surface. The schematic side-view diagram of FIG. 2 shows how light is redirected by a micro-minor 66 in the DLP spatial light modulator 60 for any of the red, green or blue color channels. An incoming laser illumination beam 18 is incident on the surface of spatial light modulator 60. The micro-mirrors 66 are tiltable in response to digital image data for an input digital image, each micro-mirror 66 being actuable to tilt with respect to a tilt axis between a first tilt position 64 and a second tilt position 65. In the first tilt position 64 the micro-mirror 66 deflects the illumination beam 18 to provide on-state modulated light 22 along an optical axis O. In the second tilt position 65, the illumination beam 18 is directed away from the optical axis O to provide the off-state dump light 23. The micro-mirrors 66 deflect the illumination beam 18 along an arced path Q defining a deflection plane 67 as they are tilted between the first tilt position 64 and the second tilt position 65.
Once the dump light 23 is generated, it must be contained and absorbed in some way to prevent it from randomly reflecting from a chassis or other surface and inadvertently leaking back into the light path used for modulated light 22. This would degrade image contrast and could also compromise color fidelity and overall image quality. As a related problem, heat is produced when intense light levels, such as those generated by light modulation assemblies 40r, 40g and 40b, are absorbed. This heat must be removed to keep the projector apparatus from overheating. To complicate the problem further, there is some unpredictability as to the amount of dump light 23 that must be handled at any one time, since this depends on the image content and can vary from frame-to-frame in a digital motion picture.
Conventional solutions for absorbing unwanted light and dissipating the heat resulting from absorption use arrangements of baffles that may be coupled with heat sink devices, further supplemented by forced air cooling in some cases. Some approaches to the problem have also attempted to combine light- and heat-absorption functions in a single component. For example, U.S. Pat. No. 6,109,767 to Rodriguez, entitled “Honeycomb light and heat trap for projector,” combines the functions of light-trapping and heat dissipation for an overhead projector by employing a coated honeycomb element in the ventilation path. U.S. Pat. No. 7,128,429 to Tausch, entitled “Light trap and heat transfer apparatus and method,” describes the use of a metallic wool material for absorbing unwanted light and heat in a curing lamp apparatus.
These approaches have been found to not provide the level of performance that is needed for the high-intensity laser light associated with a digital cinema projector. One particular problem that has not been addressed relates to the pattern of movement of the beams generated by the micro-mirror spatial light modulators 60. During the transition time interval where the micro-mirror 66 pivots between the first tilt position 64 providing the modulated light 22 and the second tilt position 65 providing the dump light 23, the reflected light traces out the arced path Q. While this transition time interval during pivoting of the micromirror is very small, on the order of fractions of milliseconds, the cumulative effect of sweeping the off state light across the arced path complicates the task of suppressing the unwanted light and resulting heat that is generated.
Thus, it can be seen that there remains a need for a solution that effectively removes the off-state light and associated heat generated in high-end digital projection systems in a simple, cost-effective manner.